The present invention relates to strain gauges, and more particularly, to a novel thin film strain gauge and the method of fabrication thereof.
Dynamic strain sensors are known to be useful for testing of mechanical components, especially when in use in their intended environment. Under certain conditions, such as in low-cycle-fatigue and flutter dynamic strain testing, associated with development of blades for aircraft engine compressors and steam turbines, strain gauges are located on critical stages of the apparatus and, being exposed to conditions of high oxidation, erosion and vibratory stress, experience unacceptable fatigue damage (to the strain sensor element and its associated conductive leads) and premature failure. Strain gauges having extended operating times, when utilized in hostile environments, will eliminate test delays and repeated sensor replacement, to result in potentially large cost savings on test programs.
One common present method of applying strain gauges to parts, such as compressor blades for aircraft engines, consists of preforming a small diameter resistance wire, typically of about 0.8 milli-inches diameter, into a grid pattern and attaching this wire grid to a surface of the part-under-test by plasma- or flame-sprayed Alumina (Al.sub.2 O.sub.3). The Alumina not only provides electrical insulation from the (generally) electrically-grounded underlying part surface (substrate), but also acts as the "adhesive" medium holding the strain gauge in contact with the substrate, whereby the strain developed in the part may be directly correlated to the change in resistance of the wire grid. At least two problems are known to be associated with this technique: the gauge wire (in order to have a sufficiently large magnitude of resistance to allow a desired accuracy of measurement) occupies a large area upon the surface of the part and the thickness of the sprayed Alumina (which thickness may be as great as 20 milli-inches) is relatively thick, to change the mass, shape and other natural mechanical characteristics of the part-under-test and to reduce the accuracy of the measurement thereon. Further, the porosity of the sprayed Al.sub.2 O.sub.3 coating both allows leakage of electrical current to the ground potential at the underlying substrate surface and facilitates oxidation and erosion leading toward premature failure of the strain gauge. Therefore, a strain gauge is desired which will not only facilitate a more reliable and accurate measure of the strain occurring in the part-under-test, but will also facilitate this measurement without significantly altering the mechanical characteristics of the part and will do so in a manner maximizing resistance to fatigue, corrosion and erosion while allowing measurement of the strain at a more localized area of the device than presently possible.